Oligomers/polymers with multiple

As enzymatic oxidative transformation of the PVA polymer can act as a multiple simultaneous event on the polymer with concurrent chain fission by the appropriate enzymes, the polymer can be broken down into small oligomers that can be channelled into the primary metabolism. This picture is not complete because PVA is usually more or less acetylated. The DH is a pivotal factor in almost every aspect of PVA application. Surprisingly there are very few data dealing with the enzymes involved in the deacetylation of not fuUy hydrolysed PVA polymer. In technical processes, esterase enzymes are widely applied to deal with PVAc structures. A good example is from the pulp and paper industry [85], where PVAc, a component of stickies , is hydrolysed to the less sticky PVA. Esterases from natural sources are known to accept the acetyl residues on the polymer as substrate but little detailed knowledge exists about the identity of acetyl esterases in the PVA degradative environment [86]. 

A repeat of the coupling and dimerization sequence with 52e produced the trimer 53. Coupling of 1,4-diiodobenzene with two equiv of 47 appeared to lead to the oligomers/polymers 54 having multiple dibenzo[a,e]cyclooctenyl units. 

Figure 2.5 Four ESI mass spectra of PEG polymers. The PEG 1000 and PEG 1450 mass spectra show distinct oligomers with multiple charge states. The PEG 8000 and PEG17500 mass spectra show unresolved distributions of oligomers and charge states. Source Author s own files...

A number of examples of oligomers and polymers containing metal-metal multiple bonds also have been reported. " Chisohn has synthesized polymers with metal-metal single and multiple bonds from bimetallic carboxylates and coordinating ligands. Scheme 24 illustrates the synfliesis of a ruthenimn coordination polymer that contains Ru-Ru double bonds. Polymer 81 was prepared from reaction of the ruthenium carboxylate monomer (79) wifli pyrazine (80). The polymer dissociated in coordinating solvents such as THF, and the MW was dependent on its concentration in noncoordinating solvents such as benzene. The polymer was thermally stable only to 140°C, at which point pyrazine was evolved. 

In this chapter we will discuss some aspects of the carbonylation catalysis with the use of palladium catalysts. We will focus on the formation of polyketones consisting of alternating molecules of alkenes and carbon monoxide on the one hand, and esters that may form under the same conditions with the use of similar catalysts from alkenes, CO, and alcohols, on the other hand. As the potential production of polyketone and methyl propanoate obtained from ethene/CO have received a lot of industrial attention we will concentrate on these two products (for a recent monograph on this chemistry see reference [1]). The elementary reactions involved are the same formation of an initiating species, insertion reactions of CO and ethene, and a termination reaction. Multiple alternating (1 1) insertions will lead to polymers or oligomers whereas a stoichiometry of 1 1 1 for CO, ethene, and alcohol leads to an ester. 

More recently, the same principle was applied by the same authors to cyclic alkanes for catalytic ring expansion, contraction and metathesis-polymerization (Scheme 13.24) [44]. By using the tandem dehydrogenation-olefin metathesis system shown in Scheme 13.23, it was possible to achieve a metathesis-cyclooligomerization of COA and cyclodecane (CDA). This afforded cycloalkanes with different carbon numbers, predominantly multiples of the substrate carbon number the major products were dimers, with successively smaller proportions of higher cyclo-oligomers and polymers. 

When ethylene reacts with triethyl- or tripropylaluminum, multiple carbometa-lation takes place, resulting in the formation of oligomers.509 Oxidation of the products followed by hydrolysis yields alcohols, whereas displacement reaction produces terminal alkenes that are of commercial importance.510 Transition-metal compounds promote the addition to form polymers (Ziegler-Natta polymerization see Section 13.2.4). 

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